This application is based on Japanese Patent Application No. 11-237298 filed on Aug. 24, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing method and a printing apparatus, and is particularly suited for adjusting the positions of ink dots in a printing apparatus of an ink jet system. In addition to general printing apparatus, the present invention can also be applied to copying machines, facsimiles with a communication system, word processors with a printer, and industrial printing apparatus combined with a variety of processing devices.
2. Description of the Related Art
An image printing apparatus of so-called serial scan type, which executes the print operation while scanning a print head, or a printing unit, over a print medium, has found a variety of image forming applications. The ink jet printing apparatus in particular has in recent years achieved high resolution and color printing, making a significant image quality improvement, which has resulted in a rapid spread of its use. Such an apparatus employs a so-called multi-nozzle head that has an array of densely arranged nozzles for ejecting ink droplets. Images with still higher resolution has now been made possible by increasing the nozzle density and reducing the amount of ink per dot. Further, to realize an image quality approaching that of silver salt picture, various technologies have been developed, including the use of pale or light color ink with reduced density in addition to. four basic color inks (cyan, magenta, yellow and black). A print speed reduction problem, which is feared to arise as the picture quality advances, is dealt with by increasing the number of print elements, improving the drive frequency and employing a bi-directional printing technique, thus realizing a satisfactory throughput.
FIG. 17 schematically shows a general construction of a printer that uses the multi-nozzle for printing. In the figure, reference number 1901 represents head cartridges corresponding to four inks, black (K), cyan (C), magenta (M) and yellow (Y). Each head cartridge 1901 consists of an ink tank 1902T filled with a corresponding color ink and a head unit 1902H having an array of many nozzles for ejecting the ink supplied from the ink tank onto a print medium 1907.
FIG. 18 schematically shows the head unit 1902H in the Z direction for illustrating representing the nozzle array thereof. In this example, ejection openings 2001 are arrayed in one line.
In FIG. 17, designated 1903 is a paper feed roller which, in cooperation with an auxiliary roller 1904, clamps a print medium (print paper) 1907 and rotates in the direction of arrow in the figure to feed the print paper 1907 in the Y direction as required. Denoted 1905 is a pair of paper supply rollers that clamp the print paper 1907 and carries it toward the print position. The paper supply rollers 1905 also keep the print paper 1907 flat and tight between the supply rollers and the feed rollers 1903, 1904.
Designated 1906 is a carriage that supports the four head cartridges 1901 and moves them in a main scan direction during the print operation. When the printing is not performed or during an ink ejection performance recovery operation for the head unit 1902H, the carriage 1906 is set at a home position H indicated by a dotted line.
The carriage 1906, which was set at the home position h before the print operation, starts moving in the X direction upon reception of a print start command and at the same time the head unit 1902H ejects ink from a plurality of nozzles (n nozzles) formed therein according to print data to perform printing over a band of a width corresponding to the length of the nozzle array. When the printing is done up to the X-direction end of the print paper 1907, the carriage 1906 returns to the home position h in the case of one-way printing and resumes printing in the X direction. In the case of bi-directional printing, the carriage 1906 also performs printing while it is moving in a xe2x88x92X direction toward the home position h. In either case, after one print operation (one scan) in one direction has been finished before the next print operation is started, the paper feed roller 1903 is rotated a predetermined amount in the direction of arrow in the figure to feed the print paper 1907 in the Y direction a predetermined distance (corresponding to the length of the nozzle array). By repeating the one-scan print operation and the print paper feeding by a predetermined distance, data for one sheet of paper is printed.
Unlike a monochromatic printing that prints only characters such as letters, numbers and symbols, the color image printing must meet various requirements such as color development, grayscale characteristic and uniformity. As to the uniformity in particular, slight variations among individual nozzles that are produced during the manufacture of a multi-nozzle head formed integrally with many nozzles (in this specification the nozzle generally refers to an ejection opening, a liquid passage communicating with the ejection opening and an element for generating energy used to eject ink) influence the amounts of ink ejected from the individual nozzles and the directions of ink ejection during printing and eventually degrade the image quality in the form of density variations of the printed image.
Detailed examples will be explained by referring to FIGS. 19A-19C, 20A-20C and 21A-21C. In FIG. 19A, designated 3001 is a multi-nozzle head with a construction similar to the one shown in FIG. 18, which is shown to have only eight nozzles 3002 for simplicity. Denoted 3003 are ink droplets ejected from the nozzles 3002. It is ideal that the ink droplets are ejected in equal amounts and in the same direction. If ink ejection is done in this manner, ink dots of equal sizes land on the print medium, as shown in FIG. 19B, resulting in a uniform density distribution with no unevenness in density (FIG. 19C).
In reality, however, individual nozzles have their own variations and if the printing is done in a manner described above, the ink droplets ejected from individual nozzles vary in size and direction as shown in FIG. 20A, forming ink dots on the paper surface as shown in FIG. 20B. From this figure it is seen that a blank part appears cyclically in the head main scan direction, dots overlap excessively in other parts, or a white line occurs at the central part in the figure. The ink dots printed in this way produce a density distribution in the direction of nozzle arrangement or nozzle column as shown in FIG. 20C, which is perceived as unevenness in density by normal human eye.
To deal with the problem of the unevenness in density, the following method has been proposed.
This method will be explained by referring to FIGS. 21A to 21C. Although the head 3001 is scanned three times as shown in FIG. 14A to complete the print in an area similar to that shown in FIGS. 19A-19C and FIGS. 20A-20C, an area of four pixels. one-half the vertically arranged eight pixels, is completed with two scans (passes). In this case, the eight nozzles of the head 3001 is divided into two halves, upper four nozzles and lower four nozzles, and the number of dots formed by one nozzle in one scan is equal to the image data culled to one-half according to a predetermined image data arrangement. During the second scan, dots are embedded at the remaining half of the image data to complete the print in the four-pixel area. This method of printing is called a multi-pass printing method. With this printing method, if a print head similar to the one shown in FIG. 20A is used, the individual nozzle influence on the printed image is halved, so that the printed image will be as shown in FIG. 21B, rendering the white lines or dark lines shown in FIG. 20B less noticeable. Hence, the unevenness in density is significantly improved as shown in FIG. 21C when compared with FIG. 20C.
While the same print area has been described to be completed in two scans, the multi-pass printing improves the image quality as the number of passes increases. This however elongates the print time, which means that there is a trade-off relation between the image quality and the print time.
Under such a situation, there have already been various proposals for the purpose how speedily and beautifully an image can be outputted. Japanese Patent Application Laid-open No. 5-31922 (1993) discloses such contents that an image data arrangement. by a tone production method such as a dither method are masked by applying a thinning pattern with dot arrangement asynchronous with the image data arrangement. According to this Application, a data printing ratio is made equal in plural passes as far as possible to obtain a smooth image by using a mask pattern which does not synchronized with a predetermined dither pattern. However, although this method has been able to cope with the predetermined dither pattern for the purpose, it has been difficult to equally cope with all the binarization methods.
Moreover, Japanese Patent Application Laid-open No. 7-52390 (1995) discloses a printing method using a mask pattern provided with randomness. According to this method, the principal object in a divided printing, i.e., improvement in unevenness of an image caused by connecting parts and variation in nozzles is possible to any binarization method.
The above-mentioned divided printing has had a problem that time and cost required for printing a sheet of paper increases as the number of division increases, and the throughput of printing decreases. To improve this problem, reduction in printing time can be considered by carrying out printing in the process of reciprocal scanning of a carriage (bi-directional printing). According to this method, since all carriage scanning operations made to return to the home position without printing anything are omitted, a printing time for a sheet of paper can be reduced approximately by half. And, in practice, the bi-directional printing has frequently been adopted as a printing method of a monochrome image.
However, in a color ink-jet printing apparatus, it has been difficult to realize bi-directional printing due to the below described factors.
FIGS. 22A and 22B show a state in which dots of a printing ink widely used at present are landed on a printing medium (paper) P, and here, the figures show the case where ink dots of different colors are absorbed (printed) at almost adjoining positions at a time interval. Here, it should be noted that in the overlapped part of the two dots, the ink dot landed later sinks into the paper deeper than the ink dot landed earlier. This fact is for the following reason. Namely, since bonding between the printing medium and a coloring matter is limited at the stage when the coloring matter such as dyestuff in the ejected ink is physically and chemically bonded to the printing medium, and the bonding between the coloring matter in the precedingly ejected ink and the printing medium is prioritized, the ink coloring matter remains on the surface of the printing medium more than the following one, so far as coloring matters do not differ much in the bonding strengths depending on the kinds. Therefore, it is thought that the ink coloring matter subsequently landed is hard to be bonded to the surface of the printing medium, and sinks in the paper in the depth direction to dye and bond it. In this case, even though two kinds of inks are landed at the same position, their colors are prioritized according to the landing order, and result in representing two different colors to visual characteristic of human eyes.
In the construction shown in FIG. 17, the four color heads 1901, which are arranged in order of black (K), cyan (C), magenta (M), and yellow (Y) from the right in this figure, move to the right as shown by the x-coordinate from the printing start position shown in the figure in the forward scanning, and perform printing operation by ejecting each ink in the moving process. Since the order of printing on the paper is in accordance with that of the above-mentioned arrangement in this case, for example, when a signal of green (cyan+yellow) is inputted for a certain area, the inks are absorbed in each pixel in order of cyan and yellow. Therefore, the cyan absorbed precedently is the prioritized color in this scanning, and green dots with a cyan tone are formed. On the other hand, in the return or backward scanning after the paper has been fed in the direction of y-coordinate, the four color heads are positioned at the right side in the figure, and then perform printing operation while moving in the reverse direction of the forward way. Therefore, the landing order is also inverted, and green dots with a yellow tone are formed in this scanning.
If scanning for printing is repeated as the above, a green dot area with a cyan tone and a green dot area with a yellow tone are alternately formed in the sub-scanning direction (y-direction) according to the forward and backward printings with the printing heads. Namely, if print-scanning is carries out without considering the divided printing and the paper is fed by an amount of the y-directional head width between the forward and backward scans, the green area with a cyan tone and the green area with a yellow tone are alternately repeated at each head width in the y direction, and this causes deterioration in quality of the green image which should be even.
However, it is possible to overcome this harmful influence a little by using the divided printing method already described above. Namely, although green dots with a cyan tone are printed in the forward scanning and green dots with a yellow tone are printed in the backward scanning, even if the divided printing is performed, the paper is fed by an amount smaller than the head width between the forward and backward scans, therefore, a color tone in a certain area contains a mixture of both tones of dots, and this relaxes unevenness of color.
These constructions and effects have already been disclosed in U.S. Pat. No. 4,748,453. This invention describes such effects that although a paper feed amount is not restricted, ink is prevented from beading on a medium such as a plastic sheet for an OHP by performing supplementary printing to the pixels alternately positioned in the horizontal and vertical directions in the printing area by printing divided into the first and second (or more) scans, and that when a color image is formed, color-banding (unevenness of color) can be prevented by inverting the landing order of the inks at mixed color pixels in the first and second scans (forward and backward printing). Since it is the principal object of the invention of the above-mentioned Patent to prevent beading between each pixel, it is characterized that each pixel printed in a single scanning is alternate in the horizontal and vertical directions (not adjoining each other).
On the other hand, according to Japanese Patent Application Laid-open No. 58-194541 (1983) applied by the applicant of the present invention, such a printing method is disclosed that when plural arrays of printing elements are arranged in parallel and main scanning of dot matrix printing is carried out by moving them forward and backward in the direction perpendicular to the array of printing elements, the duplicate printing dots by said plural arrays of printing elements have been made to differ in order of the duplicate printing on the forward and backward ways of said main scanning from each other, by intermittently printing the smaller number of dots than the entire dots to be printed at least in either of each row or column of the printing dot matrix in the forward main scanning, and intermittently printing the remaining dots in either of each row or column of the printing dot matrix in the backward main scanning. This Application does not describe either about restriction to reduce a paper feed amount smaller than a normal one as in the divided printing mentioned above, but describes, as an effect, about prevention of image deterioration caused by color tone irregularity (color banding) of a printed image based on the duplicate printing with the color inks. Moreover, since it is the principal object in the invention of the Application to prevent this color tone irregularity, no special restriction is described on dot positions to be printed in each scanning, but horizontal thinning in which dots are alternately printed only in the vertical direction and vertical thinning in which dots are printed alternately only horizontal direction are described, in addition to a checker pattern printing.
Moreover, the Japanese Patent Publication No. 63-38309 (1988) also discloses that, although this Publication is not restricted to a color printer, a construction for performing back-and-forward printing using a diced form (checker pattern) is disclosed. An object of the invention in the Publication is to prevent adjoining dots from being successively printed and to prevent dot-distortion from occurring by avoiding printing an adjacent dot before the printed dot is dried. Therefore, according to this invention, similarly to the above-mentioned U.S. Pat. No. 4,748,453, the thinning-out mask is restricted to a diced form (checker pattern).
However, even though the divided printing is performed according to the checker pattern disclosed in the Publication, the harmful influence of color irregularity is not completely solved yet.
The reason for the above will be explained below by referring to FIGS. 23A to 23C, and 24. Usually, an ink droplet quantity is designed so as to spread larger than an area given to each pixel on the paper. This is for the purpose of completely blinding a white part (ground of a printing medium) of the paper to an area of 100% data printing ratio. Therefore, when a two-divided printing is performed, a pixel itself is printed only 50% by a single scanning, but almost 100% area of the printing medium (printing paper) is covered.
FIGS. 23A and 23B show the cross sections in this case. Here, they show that the first pass (forward scanning) provides a checker pattern printing, and the second pass (backward scanning) provides an inverted checker pattern.
FIG. 23A shows the appearances of inks directly after printing in the first pass (forward scanning), and the part fully painted out is printed with cyan ink, and the shaded portion is printed with yellow ink. Since the yellow ink is landed at the same position as the cyan ink at a slight time interval, when they are absorbed in the paper, the cyan ink blurs little and the density stays high, while the yellow ink goes under and around the cyan ink, to blur large, and the density becomes low. Moreover, the absorption of these inks extends to the adjacent pixels in this case, so that the paper surface is almost filled with the inks (FIG. 23B).
In the second pass printing (backward scanning) performed under this condition, the inks are landed on the spot where the adjoining inks are already absorbed. Since the second pass is a backward scanning, the yellow ink is landed before the cyan ink (FIG. 23B). When the inks are absorbed as they are, such an absorption state is brought as both colors does not appear much on the surface in the end as shown in FIG. 23C. And, as a finally completed image, the cyan density of the first printing is emphasized most strongly, and this printing area is provided with a green image with emphasis on cyan. Contrariwise, in the printing area provided with the first pass printing by the backward scanning and adjacent to the above-mentioned printing area, the cyan and the yellow are inverted, so that a green image is obtained with a yellow tone prioritized.
FIG. 24 shows the printed states of the above. two printing areas, representing the case in which forward and backward printing was carried out by using a multi-nozzle head with 16 nozzles according to the method described in FIGS. 21A to 21C. From this figure, it can be seen that a precedent half of the head always determines a prioritized color for each area of an eight-dot width. and that the prioritized colors are inverted from each other in forward and backward scanning. Although the description has been made here assuming that a checker patterned mask was applied, a random mask printing method disclosed in the before-mentioned Japanese Patent Application Laid-open No. 7-52390 (1995) has brought a similar result, and since two areas with different prioritized colors existed alternately, color irregularity has still appeared also in divided printing and deteriorated an image, and a bi-directional printing has been made difficult.
As a solution of the harmful effect of the above-described color irregularity, the invention disclosed in Japanese Patent Application Laid-open No. 6-22106 (1994) is mentioned. According to this invention, a group of mxc3x97n pieces of pixels is used as a unit for printing, and printing is performed by using an arranging matrix in which the groups are not adjacent to each other. The Application discloses such an effect as an amount swelling out to a blank paper area has been reduced by printing a batch of mxc3x97n pieces of pixels, and a difference between prioritized colors in forward and backward printings has been eliminated to reduce the harmful effect of color irregularity.
However, the technique disclosed by the above-described Japanese Patent Application Laid-open No. 6-22106 (1994) has not suited for printing a high quality photographic tone image which is required nowadays. The reason is that a batch of gathered dots unit for a sufficient effect of controlling color irregularity exceeds a degree of human visual resolution, so that a texture becomes visually conformable. Namely, although the invention has been able to fully cope with a desktop publishing, and a printing for images consist of graphics or text, it could be harmful to a photographic image quality which is in great demand in recent years. Therefore, under the existing circumstances, it is general to cope with the problem by applying a random mask printing method with increasing the number of divided printing, however, when bi-directional printing is carried out to increase a throughput, color irregularity still remains to some extent and the problem is still not fully coped with.
The present invention is made considering the above-mentioned problem, and the purpose is to make it possible to form a high quality image at a high speed, namely, to make it possible to print a photographic image without color irregularity at a high speed.
In a first aspect of the present invention, there is provided a printing method using a print head on which a plurality of ejection openings for ejecting inks are arranged, the method comprising the steps of:
moving the print head to scan in forward and backward directions different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in the direction perpendicular to the scan directions by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by the forward and backward scans at plural times in accordance with pixel arrangements in a complementary relation to the same image area; and
making the sum of ratios of the data print quantity in the odd-numbered scans among the plural times of the forward and backward scans to the total data print quantity to the same image area, smaller than the sum of ratios of the data print quantity in the even-numbered scans among the plural times of the forward and backward scans, during the forming of image.
In a second aspect of the present invention, there is provided a printing apparatus using a print head on which a plurality of ejection openings for ejecting inks are arranged, the apparatus comprising:
means for moving the print head to scan in forward and backward directions different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in the direction perpendicular to the scan directions by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by the forward and backward scans at plural times in accordance with pixel arrangements in a complementary relation to the same image area; and
means for making the sum of ratios of the data print quantity in the odd-numbered scans among the plural times of the forward and backward scans to the total data print quantity to the same image area, smaller than the sum of ratios of the data print quantity in the even-numbered scans among the plural times of the forward and backward scans, during the forming of image.
In the first or second aspect of the present invention, the ratio of the data print quantity in a first scan may be made smaller than the ratio of the data print quantity in a second scanning, among the plural times of the forward and backward scans.
The scans may be carried out three times or more to the same image area, and the ratio of the data print quantity in a third scan and thereafter may be made larger than the ratio of the data print quantity in a first scan and smaller than the ratio of the data print quantity in a second scan.
Here, the sum of covering ratios of the printing medium by formed dots in the first scan and the second scans may be made larger than 50%.
In the above, the print head may have plural arrays of the plurality of ejection openings side by side in the scan directions corresponding to inks with different color tones.
The pixel arrangement in at least a first scanning among the plural times of the forward and backward scans may be specified in a unit of mxc3x97n (n, m: integer) pixels.
In a third aspect of the present invention, there is provided a printing method using a print head on which a plurality of ejection openings for ejecting inks are arranged, the method comprising the steps of:
moving the print head to scan in forward and backward directions different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in the direction perpendicular to the scan directions by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by the forward and backward scans at plural times in accordance with pixel arrangements in a complementary relation to the same image area; and
making a ratio of the data print quantity in a first scan among the plural times of the forward and backward scans to the total data print quantity to the same image area smaller than a ratio of the data print quantity in a second scanning, and making the sum of covering ratios of the printing medium by formed dots in the first scan and the second scans larger than 50%, during the forming of image.
In a fourth aspect of the present invention, there is provided a printing apparatus using a print head on which a plurality of ejection openings for ejecting inks are arranged, the apparatus comprising:
means for moving the print head to scan in forward and backward directions different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in the direction perpendicular to the scan directions by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by the forward and backward scans at plural times in accordance with pixel arrangements in a complementary relation to the same image area; and
means for making a ratio of the data print quantity in a first scan among the plural times of the forward and backward scans to the total data print quantity to the same image area smaller than a ratio of the data print quantity in a second scanning, and making the sum of covering ratios of the printing medium by formed dots in the first scan and the second scans larger than 50%, during the forming of image.
In the third or fourth aspect of the present invention, the covering ratios on the printing medium by formed dots in the first scan and the second scan may be substantially equalized.
In a fifth aspect of the present invention, there is provided a printing method using a print head on which a plurality of ejection openings for ejecting inks are arranged, the method comprising the steps of:
moving the print head to scan in forward and backward directions different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in the direction perpendicular to the scan directions by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by the forward and backward scans at plural times in accordance with pixel arrangements in a complementary relation to the same image area;
a first control step of making the sum of ratios of the data print quantity in the odd-numbered scans among the plural times of the forward and backward scans to the total data print quantity to the same image area, smaller than the sum of ratios of the data print quantity in the even-numbered scans among the plural times of the forward and backward scans, during the forming of image; and
a second control step of performing a control similar to the first control step except for making the number of the plural times of the forward and backward scans less than in the first control step, and also specifying the pixel arrangement at least in a first scan among the plural times of the forward and backward scans in a unit of mxc3x97n (n, m: integer) pixels.
In a sixth aspect of the present invention, there is provided a printing apparatus using a print head on which a plurality of ejection openings for ejecting inks are arranged, the apparatus comprising:
means for moving the print head to scan in forward and backward directions different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in the direction perpendicular to the scan directions by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by the forward and backward scans at plural times in accordance with pixel arrangements in a complementary relation to the same image area;
a first mode for making the sum of ratios of the data print quantity in the odd-numbered scans among the plural times of the forward and backward scans to the total data print quantity to the same image area, smaller than the sum of ratios of the data print quantity in the even-numbered scans among the plural times of the forward and backward scans, during the forming of image; and
a second mode for performing a control similar to the first mode except for making the number of the plural times of the forward and backward scans less than in the first mode, and also specifying the pixel arrangement at least in a first scan among the plural times of the forward and backward scans in a unit of mxc3x97n (n, m: integer) pixels.
In the fifth or sixth aspect of the present invention, switching between the first and second control steps or modes may be possible.
In the above, the print head may have heating elements to generate thermal energy for causing film boiling in ink as an energy for ejecting ink from the ejection openings.